Signal monitoring circuits are used in many applications involving the protection of circuitry and the control of circuit functions. A specific signal monitoring application requiring a rapid response of the monitoring apparatus is the field of uninterruptible power supplies where two power sources are coupled to a power control unit which supplies a continuous power output irrespective of the failure of one or the other power sources. In general, the power supply power output depends basically on a commercial AC power source which operates continuously and a secondary power source is held in reserve to become operative and supply power upon the failure of the basic commercial AC power source. In order to maintain the power output truly continuous, the failure of the commercial power source must be ascertained rapidly so that the secondary or reserve source may be promptly switched to supply the output power.
One of the signal failures of the commercial power source which must be monitored is the occurrence of short circuits which rapidly drop the voltage to zero. Conventional short circuit detectors normally operate by monitoring either the average of the input signal or the periodic attainment of a minimum threshold signal magnitude. Both of these methods are limited in the rapidity of their response. Averaging is time consuming and the detection of a threshold requires that the monitoring circuitry take into account the periodic time interval necessary to periodically obtain that threshold. This periodic time is normally a substantial part of the periodic cycle and hence limits the speed of response of the monitoring circuitry. A simple instantaneous failure check of the magnitude of a periodic signal being monitored is not suitable for activating such a monitoring circuit since in the typical commercial periodic signal a zero crossing occurs twice in each cycle.